The present invention relates to the engine oils formulated the meet the new specifications of ILSAC GF-3 using conventional high sulfur base stocks.
Lubricating base stocks or base oils have been categorized into Groups I-V by the American Petroleum Institute (API). They are characterized by their sulfur content, xe2x80x9csaturatesxe2x80x9d or paraffin content and their viscosity index. Group I and II are most abundant base stocks and most commonly used to formulate engine lubricants. Group I base stocks are typically much higher in sulfur content than Group II. The trend is toward Group II base stocks which are both low in sulfur and low in aromatic content. The problem presented to the lubricating oil additive supplier is to formulate a performance additive package for both types of oils, often for the same customer. The performance of a lubricant is demonstrated by passing engine tests (ASTM sequence tests.) The engine testing is a costly investment for the additive supplier. The Sequence IIIF test, a General Motors developed test that measures oxidation and wear among other properties, is one of the engine tests that is sensitive to the starting base oil. An additive package formulated to pass the IIIF test in Group II base stocks may not be adequate for Group I base stocks. This results in multiple formulations and engine testing and will often require two separate additive packages. This is not always economical or convenient for the additive supplier or the customer, the lubricating oil manufacturer.
Lubricating oils containing molybdenum compounds like molybdenum dithiocarbamates (MoDTCs) have been known in the literature. Le Suer in U.S. Pat. No. 3,541,014 showed the value of using oil soluble molybdenum compounds in lubricants in combination with other additives to improve extreme pressure capabilities and antiwear properties of the lubricants. Papay et al. in U.S. Pat. No. 4,178,258 discloses a lubricating oil composition suitable for use in an internal combustion engine, comprising a major amount of a mineral oil and a minor wear and friction reducing amount of an oil soluble molybdenum dithiocarbamate (MoDTC). Inoue et al. in U.S. Pat. No. 4,529,526 discloses a lubricant consisting essentially of a base oil, a molybdenum dithiophosphate (MoDTP) or a MoDTC, a zinc dithiophosphate with at least 50% secondary alkyl groups, an overbased calcium sulfonate and a succinimide dispersant or boron derivative thereof. In U.S. Pat. No. 4,846,983 Ward teaches the production of a molybdenum dithiocarbamate (MoDTC) based on a primary amine and further showed the value in using the MoDTC as an oxidation inhibitor in a Oldsmobile Sequence IIID test, a second generation predecessor of the Sequence IIIF test.
Several patents teach the use of molybdenum compounds in engine oil for use in low sulfur and/or low aromatic content base oils, often in combination with other oxidation inhibitors. For example U.S. Pat. No. 5,281,347 to Igarashi et al. shows the use of a MoDTC in a hydrocracked oil with a sulfur content of less than 50 parts per million (ppm) and a aromatic content of 3-15%. U.S. Pat. No. 5,605,880 to Arai et al. claims the combination of a MoDTC and an aromatic amine in a base oil which has less than 50 ppm sulfur and less than 3% aromatic content.
The present invention solves the problem of oxidation in high sulfur (greater than 300 ppm) Group I base stocks by top treating (i.e., supplementing) an additive formulation designed for Group II base stocks with a MoDTC. This top treatment is shown to be effective in the Sequence IIIF engine test. The base formulation designed for Group II base stocks contains a high molecular weight succinimide dispersant, an overbased calcium sulfonate, a zinc dithiophosphate and other oxidation inhibitors such as a sulfurized olefin, a hindered phenol, or an alkylated diphenylamine. The invention is particularly suitable for use with lubricant formulations in high sulfur Group I base stocks which fail the Sequence IIIF oxidation test without the added MoDTC.
The present invention provides a composition suitable for qualifying as an ILSAC GF-3 engine lubricating oil comprising the following components: a major amount of a mineral oil classified as an API Group I base stock, wherein said base stock contains 300 ppm or more of sulfur by weight, a molybdenum dithiocarbamate in an amount to deliver 25-600 ppm of molybdenum to the finished engine oil, a succinimide dispersant having a polyolefin backbone, where the polyolefin has a number average molecular weight of at least 1300, a zinc dialkyldithiophosphate derived from at least one secondary alcohol, and at least one oxidation inhibitor selected from the group consisting of hindered phenols, alkylated aromatic amines, and sulfurized olefins.
The present invention further provides a method for inhibiting oxidation in an ASTM Sequence IIIF test when using high sulfur API Group I base stocks, comprising the following steps: treating a high sulfur Group I base stock with an additive package which is capable a passing a Sequence IIIF test when formulated in Group II base stocks; and adding to said base stock a MoDTC in an amount suitable to deliver 25-600 ppm of molybdenum to the finished lubricating oil.
The additive package capable of passing the Sequence IIIF test typically comprises a succinimide dispersant having a polyolefin backbone, where the polyolefin has a number average molecular weight of at least 1300; a zinc dialkyldithiophosphate derived from at least one secondary alcohol; and at least one oxidation inhibitor selected from the group consisting of hindered phenols, alkylated aromatic amines, and sulfurized olefins.